1. Field of the Invention
The invention relates to electrically controllable optical transmission cells, such as Kerr cells, particularly with respect to the use of such cells as optical shutters. In particular, Lanthanum doped Lead Zirconate Titanate (PLZT) Kerr cells are involved.
2. Description of the Prior Art
PLZT Kerr cells are utilized as the active element in present day high speed electro-optical shutters. A Kerr cell is a transparent ceramic element with electrodes disposed thereon providing variable optical retardation as a function of applied electric field. Application of an electric field transverse to the cell optical axis varies the rotation of the polarization axis of the cell. Thus, when a Kerr cell is superposed between two crossed polarizers, light transmission through the assembly is a function of the electric field applied to the Kerr cell because of the polarization rotation in the retardation plate. In the off state of the device, with no electric field applied to the PLZT, no light passes therethrough because the axes of the polarizers are orthogonal with respect to each other. Energizing the PLZT rotates the polarization of the light passing through the first polarizer by 90.degree. causing the polarization axis of the light to align with the axis of the second polarizer. Thus, in the energized state, the optical energy is transmitted through the assembly. The proportion of optical energy transmitted in the ON state compared to that transmitted in the OFF state is denoted as the extinction ratio. The Kerr cell shutter can be placed in a continuous light transmitting state by continuous application of an activating voltage. The shutter can also be operated in a pulsed mode by application of voltage pulses thereto.
In the prior art, a voltage level or voltage pulses of the same polarity are applied to activate the device. With continuous application of a unipolar voltage or after repeated pulsing of the Kerr cell with a unipolarity pulse, a degradation in the optical retardation properties results in a relatively short time. This is due to a tendency of the optically active material in the Kerr cell to acquire an internal space charge so that the cell performs as if the activating electric field is still applied even after the activating voltage source is removed. This property has been observed when utilizing PLZT as the optically active material. This effect is more pronounced for repeated pulse activation than for continuous voltage. With the charge buildup, some optical retardation remains when the device voltge is removed. The effect in the prior art devices was that optical transmission would not be completely extinguished between the two polarizers and therefore the extinction ratio was severely degraded. In prior art devices, continuous usage for 20 hours or after application of several hundred pulses, rendered the devices useless.
Presently, there is no known way of removing the deleterious space charge except heat annealing and there is no prior method to avoid the space charge. When such prior art devices exhibited this behavior, the devices would be permitted to remain unenergized for several days. If the performance degradation did not improve, the cell would be discarded. Thus, removal from service or discarding due to space charge buildup were the only solutions to this problem in the prior art.